Extreme lower jaw elongation in a placoderm reflects high disparity and modularity in early vertebrate evolution

Jaws are a key vertebrate feature that arose early in our evolution. Placoderms are among the first jawed vertebrates; their fossils yield essential knowledge about the early diversification of gnathostome feeding strategies, diets and modularity. Modularity can be expressed through disproportional lengths of lower and upper jaws as in swordfish or halfbeaks. Alienacanthus malkowskii is an arthrodire from the Famennian of Morocco and Poland, whose most remarkable feature is its lower jaw, which is twice as long as the skull. This is the oldest record of such extreme jaw elongation and modularity in vertebrates. The gnathal plates of Alienacanthus possess sharp, posteriorly recurved teeth that continue anterior of the occlusion in the inferognathals. The dentition suggests a catching and trapping live prey function, and the jaw occlusion is unique among placoderms. This armoured ‘fish’ expands the morphological and ecological diversity during one of the first radiations of jawed vertebrates with a combination of features so far unrecorded for arthrodires.

Jaws are a key vertebrate feature that arose early in our evolution.Placoderms are among the first jawed vertebrates; their fossils yield essential knowledge about the early diversification of gnathostome feeding strategies, diets and modularity.Modularity can be expressed through disproportional lengths of lower and upper jaws as in swordfish or halfbeaks.Alienacanthus malkowskii is an arthrodire from the Famennian of Morocco and Poland, whose most remarkable feature is its lower jaw, which is twice as long as the skull.This is the oldest record of such extreme jaw elongation and modularity in vertebrates.The gnathal plates of Alienacanthus possess sharp, posteriorly recurved teeth that continue anterior of the occlusion in the inferognathals.The dentition suggests a catching and trapping live prey function, and the jaw occlusion is unique among placoderms.This armoured 'fish' expands the morphological and ecological diversity during one of the first radiations of jawed vertebrates with a combination of features so far unrecorded for arthrodires.

Introduction
Placoderms are the phylogenetically earliest jawed vertebrates [1].They play an important role for the understanding of the origin, early evolution and diversification of structures such as jaws and teeth.In the Devonian, placoderms have the highest diversity among jawed fishes and probably occupied many different habitats [2,3].In particular, arthrodires are the most common and diverse placoderm group.They display a high disparity through time, providing a record of the functional and ecological variation of an early jawed vertebrate evolutionary radiation [4][5][6].For example, the recent discovery of a selenosteid preserving its body outline [7] provided new insights into its ecology and swimming abilities.However, body contours and stomach contents are rarely preserved in arthrodires [7][8][9][10] and, among the typically preserved armoured plates of the group, gnathal ( jaw) elements yield the most information regarding ecological diversity, including feeding.Early placoderms show little jaw variation compared to later forms as jaws from the earliest taxa were adapted to fast closure and stress resistance [10].The Late Devonian presents a much higher diversification of jaw morphology, most of which favoured a variety of feeding strategies over fast closure [11,12], including filter feeding [13] and durophagous diets [14][15][16].
The fossil presented here is that of an arthrodire first described as a putative placoderm [17,18] from the Late Devonian of Poland.Two nearly complete skulls with jaws were discovered in the eastern Anti-Atlas of Morocco in a similar stratigraphic position, enabling us to better describe the species and to evaluate the phylogenetic position of Alienacanthus.We discuss jaw modularity and possibilities regarding functional morphology and feeding ecology of this taxon.We compare the jaw articulation and structures of the hyoid apparatus to that of other early vertebrates to improve our understanding of their use in this animal with unusual jaw morphology.

Preparation
The material stored in Paris was prepared by formic acid dilution and additional mechanical preparation was performed with needles.Intermediate steps in the preparation process were photographed to record the natural contacts between the dermal plates.The material stored in Zurich was mechanically prepared with a combination of air-scribes and sandblaster.Two small portions of the right inferognathal (IG) and the tip of the left one were reconstructed in the small lower jaws.Thin sections of gnathal and dermal shield elements were performed in the Muséum National d'Histoire Naturelle, Paris for their material and in the University of Warsaw for the Polish specimens.

Phylogenetic analysis
The character matrix is based on the 98 characters for 28 taxa in Jobbins et al. [7], to which six taxa obtained from Trinajstic & Dennis-Bryan [22] were added: Pachyosteus, Panxiosteus, Janiosteus, Plourdosteus, Protitanichthys and Fallocosteus.The character matrix (electronic supplementary material, table S2) was converted into a nexus file using MESQUITE [23].Dicksonosteus, Holonema, Buchanosteus and Homosteus were defined as outgroups, following Rücklin et al. [24], to determine character polarity.As in Jobbins et al. [7], characters 4, 14, 20, 35, 51, 75, 92 and 93 were ordered, as these were changing sequentially across a gradient of states.The analysis was performed using a parsimony heuristic search in PAUP [25] with the random addition sequence of 10 repetitions and the holding 100 trees-option.All trees were rooted using the 'make outgroup paraphyletic with respect to ingroup' option.Description of the characters is provided in the electronic supplementary material, info.S2.    1a-c].

Abbreviations
Remarks.The preservation of several features is the basis for an emended diagnosis (see the electronic supplementary material, info.S1 for full description of the features), because the original diagnosis was based solely on isolated material, described as '…extremely large, paired and unpaired spines, built of osseous tissue' [17, p. 357].In fact, the supposed 'spines' are IGs [18].The Moroccan material (figure 2; electronic supplementary material, figures S1 and S2) has the same age as the new Polish material, which is from the same locality and strata as the type material (electronic supplementary material, figures S3 and S4).The Moroccan and Polish IGs have the same shape.The holotype and the new specimens present an elongate and slender jaw with posteriorly recurved teeth.The curvature of the teeth is less pronounced in the holotype because they are worn, which is also seen in worn teeth at the anterior parts of the gnathal plates from Morocco.Recently discovered material from the type locality displays the same tooth curvature.The IG shape is similar in cross section throughout the plate in both the Polish and the Moroccan material, including the holotype.An inferognathal alone provides only little insight into the anatomy of the animal.Thus, we assign the new material to the type species and designate the two best-preserved specimens as epitypes for a better species referral.
Holotype Emended diagnosis.Large eubrachythoracid with long protruding IGs that bear a series of in-line teeth on the labial edge of the jaw, which are posteriorly recurved.The IGs reach about twice the length of the skull including the superognathals and lack traces of attachment for the mentomeckelian cartilage.Both superognathals have a vertical dorsal process instead of horizontal and have sharp recurved teeth posteriorly and worn teeth anteriorly.The ASGs protrude shortly anterior of the rostrum.The dermal plates of the skull-roof show no ornamentation.The rostral plate is pronounced, deprived of the laterally developed wings anteriorly, and its simple non-jagged contact with the pineal plate separate the preorbitals.The preorbital and central plates are of subequal length.The central plates only join at a single contact point with the pineal and nuchal plates.The orbits are large, and the suborbital plates possess a well-developed linguiform process.The embayment of the nuchal plate is deep and therefore the nuchal gap is probably large.

Skull
The epitypes PIMUZ A/I 5239 (figure 2a-c) and MCD 201 (electronic supplementary material, figures S1 and S2) provide data about most cranial elements, including the paired preorbital, postorbital, suborbital, central, marginal and paranuchal plates, the single rostral, pineal and nuchal plates, gnathal plates, parasphenoid and hyoid elements (detailed description in the electronic supplementary material, info.S1).The cranium is funnel-shaped with a rounded cross section throughout.Measurements of each specimen are provided in table 1.The orbits are large and drop-shaped with the tip pointing posterodorsally.The proportions of the orbits are reminiscent of those of selenosteids, a derived group of pachyosteomorphs with large eyes.

Skull plates
The rostral and pineal plates separate the preorbitals (PIMUZ A/I 5239; figure 2).There is a single, quadruple contact point between the pineal, both central, and nuchal plates.The nuchal plate is large, and its posterior margin is strongly concave, implying a large nuchal gap as in the selenosteids  Draconichthys [26], Microsteus [27] or Gymnotrachelus [28].A large nuchal gap implies the presence of well-developed nuchal muscles involved in movements raising the head [29,30].The orbital margin is formed by the preorbital, postorbital, suborbital and marginal plates.The paranuchal plates are rather small and have a short posterior pitline groove near the posterior edge.The suborbital plates are slender, and have a well-developed linguiform process, which articulates with the autopalatine.

Gnathal plates
The Moroccan specimens preserve all three paired elements comprising the jaws: the IG (lower jaw), and the anterior and posterior superognathals (PSGs; upper jaws).The IGs are prognathous; they are about twice as long as the skull.The occlusal surface extends a little anteriorly to the rostrum.The upper elements (superognathals) are straight and flattened, and the dorsal processes are on a similar dorsoventral axis as the rest of the elements, a unique feature among placoderms (electronic supplementary material, figure S5).All gnathal plates bear posteriorly recurved teeth.The posterior ones are slender and sharp, with very little gradual wear, while the anterior ones are abruptly worn.
The ASG is a solid laterally flattened plate with a thin occlusal margin and dorsal process forming a laterally acute angle with the occlusal margin.The tip of the ASG is pointed, as is that of the IGs.The PSG is slender, laterally flattened, and has a short and dorsally developed dorsal process.The IGs are the best preserved and most commonly found elements of Alienacanthus.The IG is a long and slender bone with a sharp anterior tip, Y-shape in dorsoventral view, and tooth row extending past the occlusal region.In arthrodires, the IG usually comprises a biting division (anterior) and a blade, or internal shaft ( posterior).Alienacanthus presents an additional primarily toothless section anterior to the biting division.The right and left elements were anteriorly in close contact over ca 60% of the jaw length, presenting a general jaw morphology reminiscent to other long jawed animals like mixosaur ichthyosaurs [31] or Hemiramphidae (halfbeaks) [32].A depression on the medial side of the IGs occurs where the elements meet posteriorly (figure 2g).The smooth tip of the IG of PIMUZ A/I 5238 does not show traces of contact with the mentomeckelian cartilage.

Histology of the gnathal plates
Two ossifications are discerned in the IG (figure 3).The first holds the teeth (biting division) and lies laterally, with slight elevation, next to the second (internal shaft).Both ossifications have a dense, poorly vascularized, outer layer surrounding a well-vascularized trabecular bone (figure 3c).The bones meet centrally to form a fused margin but there is no clear separation between both ossifications (figure 3d).Bone cell lacunae are present in the interstices between the osteons of the dense bone layer, particularly in the medial part (figure 3e).The cell lacunae are rounded and connected through canaliculi.The superficial medial layer also yields cracks at the interstitial zone (figure 3f ), especially in the least vascularized part of the layer; these reach into the first few outer osteon concentric lamellae and are perpendicular to the osteon margin.The teeth are ankylosed to the  3a,b).There is no presence of enamel.

Hyoid elements
Both Moroccan skulls, PIMUZ A/I 5239 and MCD201, preserve structures interpreted as the first branchial arch of the hyoid apparatus (figure 2; electronic supplementary material, figure S2).
A partially visible putative basihyal is located where both IGs meet in the first epitype, exhibiting a ventral deep pit.The ceratohyal lies next to the latter; it is triangular and surrounds the margins of the IG laterally and labially.An oblong segment branches from the posterior end of the triangular section of the ceratohyal in the second epitype.

Phylogeny and Alienacanthus affinities
The parsimony analysis of the character matrix (see Materials and methods) resulted in seven equally most parsimonious trees with a tree length of 447 steps, consistency index = 0.313 and retention index royalsocietypublishing.org/journal/rsosR. Soc.Open Sci.11: 231747 (RI) = 0.566 (electronic supplementary material, figure S6A).A strict consensus and a 50% majority-rule consensus tree were computed using these trees (figure 4; electronic supplementary material, figure S6B).In all trees, Alienacanthus is placed within Selenosteidae and, in the majority-rule consensus and four of . is placed at the base of a polytomy with Amazichthys, as a derived form within the Selenosteidae.In the three trees where Alienacanthus plots with Amazichthys as monophyletic, characters (5), ( 6) and ( 15) support the node.One tree plots Alienacanthus as sister group to the derived Moroccan selenosteids with characters ( 5), ( 6) and (96) supporting the node.In the other three trees, Alienacanthus plots with Selenosteus, a more basal form of the family (electronic supplementary material, figure S6).This affinity presents a node supported by the characters ( 6), ( 8), ( 13), (30), and (61).This uncertainty regarding the affinities of Alienacanthus might be owing to the incompleteness of the material, i.e. the lack of a preserved thoracic armour.It is possible that additional characters will contribute to the improvement of the family's phylogeny in the future as previous phylogenies have also shown some instability, demonstrated by variation within taxa placements in the family tree combined with an RI always above 0.5 [7,24,26].

Extreme jaw elongation and modularity through time
The concept of modularity argues that, within a system, components can evolve independently from each other.Based on this concept, one would argue it would be difficult for many skull elements to be modular.For instance, selenosteids possess large orbits, a feature that involves changes in multiple skull bones [26].The jaws, however, particularly the lower jaw, are elements that can change independently from the rest of the skull.The extreme lower jaw elongation in the Late Devonian placoderm Alienacanthus (figure 5) demonstrates the ancient evolvability of upper-lower jaw modularity and provides an example of the singular occupation of morphospace at the end of the 'age of fishes' [12,37].The conservation of molecular pathways and interactions involved in skeletal development across vertebrates [38] permits the reconstruction of developmental evolution in fossils [39,40].The study of placoderms has been key in royalsocietypublishing.org/journal/rsos R. Soc.Open Sci.11: 231747 understanding the early evolution and evolvability of neural crest derived structures, including teeth and jaws [41,42].Past research on jaw length regulation suggests that the anatomy of Alienacanthus most likely resulted from derived modifications in one or multiple pathways involving the transforming growth factor-beta, the bone morphogenetic proteins, the sonic hedgehog pathway members or the fibroblast growth factor [43][44][45].In halfbeak fishes specifically, the calmodulin molecule paralogue calm1, known to play a generic role in bone cell proliferation, was identified as a potential regulator of jaw length, with its differential expression in upper and lower jaws [46].This led to the development of the toothless protrusion in the lower jaw.Although Alienacanthus has no teeth on the anteriormost section of the IGs, a portion of the tooth-bearing component remains present past the occlusion with the upper gnathal elements.In any case, Alienacanthus shows an extreme dissociation in the length of upper and lower jaws, documented very few times across about 440 million years of jawed vertebrate evolution.The extreme extent of the lower jaw elongation independent of the upper jaw is similar to that of three marine vertebrate taxa: the Carboniferous chondrichthyan Ornithoprion [47]; the ray-finned halfbeak fishes (Hemiramphidae) including their fossil record extending back to the Palaeogene [32]; and the Pliocene porpoise Semirostrum [48] (figure 4).Respectively, the lower jaws of these three taxa are 1.3, 1.6, and 1.2 times the length of the skull, less than the ratio of 2 for Alienacanthus.The two IGs of Alienacanthus were not fused in contrast to the lower jaws of the other taxa with extreme lower jaw elongation.The presence of a depression on the medial side of the IGs (figure 2g) suggest that there may have been some ligament or cartilage attached in the area to reinforce jaw connection.It is likely that the elements were in contact or near contact anterior to this attachment.In addition, the animals mentioned above date back to the Palaeogene (halfbeaks), Pliocene (Semirostrum) and Carboniferous (Ornithoprion).This makes Alienacanthus the earliest vertebrate displaying such elongation of the lower jaw.

Jaw morphology and anatomy
The unique jaw configuration of Alienacanthus must have resulted in species-specific behaviours including feeding, for which we can offer hypotheses based on the anatomical information derived from the several fossils reported here.
The unusual and extreme shape of the gnathal elements and their position in the articulated specimens suggest that the occlusion occurred only laterally without an anteriorly enlarged occlusion surface (electronic supplementary material, figure S5).This is a feature unique to Alienacanthus as arthrodiran placoderm jaws are normally rounded anteriorly (both ASG and IG), forming an occlusion laterally and anteriorly (i.e.U-shaped) (e.g.[26,35,[49][50][51]).In Alienacanthus, the occlusion of the upper with the lower jaws continues anteriorly past the rostral plate and the ASGs do not meet.This is because the IGs are medial when occluding with the superognathals, thus preventing a contact between the ASGs anteriorly.Regarding the upper jaws' lateral articulation, the orientation of the dorsal process in the ASGs and PSGs is different to what has been documented so far in arthrodires.The dorsal lamina is in contact with regions of the ethmoid part of the neurocranium (for the ASG), the autopalatine (for the PSG), and a part of the palatoquadrate [30].The dorsal processes of the upper gnathal plates in arthrodires, where present, are horizontal, forming an internal open near 90°angle with the occluding surface of the gnathal plates.In Alienacanthus, the processes are on the same axis as the shearing surface, making it unique to what was known so far in placoderms (e.g.[8,14,26,[51][52][53]) (electronic supplementary material, figure S5C).This unique articulation also suggests that there was some flexibility in the movement of the ASGs medially.
As in other arthrodires like Compagopiscis [54] or Plourdosteus [55], there are two ossifications in the IG of Alienacanthus: the internal shaft and the tooth-bearing bone (figure 3).There are, however, key features that differ between the former and the latter.The last added tooth is the most posterior one in Alienacanthus, which implies tooth addition occurred posteriorly only.This is different from other derived forms like Compagopiscis, in which the dental ossification growth occurs in anterior, posterior and lingual directions [54,55].Additionally, the margin between the tooth-bearing bone and the internal shaft is tight, i.e. the external border of outer layers forms only one dense layer, and the biting region does not surround the shaft ventrally and dorsally.By contrast, Compagopiscis and Plourdosteus show a distinct delimitation between the two ossifications' outer compact layer at their contact margin and the biting division surrounds the internal shaft ventrally, laterally and dorsally.Moreover, in the thin section, the trabecular bone of Alienacanthus is spongier than that of Compagopiscis and Plourdosteus.
All gnathal elements bear posteriorly and slightly medially recurved teeth.This is a feature also seen in the enigmatic aspinothoracids Diplognathus mirabilis [56,57] and Diplognathus lafargei [57] from the royalsocietypublishing.org/journal/rsos R. Soc.Open Sci.11: 231747 Cleveland shale of Ohio.The jaws comprise long and slender IGs that form a fork-like symphysis at the tip, and possess posteriorly recurved teeth.Like Alienacanthus, Diplognathus is a rare taxon showing a unique jaw morphology with a large portion of the bone occupied by the occlusal region (over 50%).Other placoderms with recurved teeth include the selenosteid Draconichthys elegans, although these are recurved lingually only [26], and the dinichthyid Hadrosteus rapax with anteriorly recurved teeth [58,59].

Alienacanthus versus other arthrodire feeding strategies
The recurved teeth of the gnathal plates suggest that they functioned to capture live prey, such as fishes, and to direct it towards the oesophagus, as in many reptiles like snakes [60], pliosaurs [61] and choristoderans [62], or in fishes like the bowfin fish [63], northern snakehead or Atlantic salmon [63].Another remarkable characteristic of the lower jaw is the continuation of the teeth beyond the occlusion.Several teeth lie anterior to this surface, ca 12 in the incomplete IG of PIMUZ A/I 5239.The presence of teeth anterior to the occlusal surface is seen in the ichthyosaurs Eurhinosaurus [64] and Excalibosaurus [65], as well as in two extant and one extinct chondrichthyan groups, the Pristiophoridae (sawsharks), the Pristidae (sawfish/carpenter sharks) and the Sclerorhynchoidei (rajiform ray).In these taxa, this feature lies in the upper jaw or rostrum.In the chondrichthyan groups, their rostra are equipped with sensory organs that allow detection and monitoring of movement near the animal through electroreception [66][67][68], although this is not limited to elongated jaws.Their teeth are used as a tool with the rostrum to strike the prey after detection [68].Micro-teeth were reported and studied in three billfishes (Kajikia audax, Istiophorus platypterus and Makaira nigricans), for which they were also used to perform high speed dashes or precise strikes to attack prey of various sizes (large prey for Makaira and smaller prey from schools for Kajikia and Istiophorus) [69].Electro-sensitivity was also suggested to be present in the lower jaw of Semirostrum [48] and, phylogenetically closer, in the elongated rostrum of the placoderm Carolowilhelmina [70].Thin sections of Alienacanthus in the protruding section of the jaw do not indicate the presence of specialized nerve canals related to such function.This suggests that there was no increased sensitivity in its lower jaw.Semirostrum and halfbeaks bear a jaw that protrudes in a different way than in Alienacanthus; both have a normal length occlusal surface, i.e. the extension is edentulous, like a long chin.Doryaspis is a Devonian heterostracan ( jawless vertebrate) with a long ventral pseudorostrum, comparable to the chin in gnathostomes [71][72][73].Specifically, Doryaspis nathorsti [72,73] is equipped with small hookshaped tubercles on the pseudorostrum's lateral margins resembling teeth.Alienacanthus has teeth that continue past the occlusal surface in addition to jaw elongation.We suggest that the anteriormost teeth were part of the occlusion at an earlier ontogenetic stage but, with growth of the IGs, these teeth moved out of the occlusal surface.We propose either a sudden growth of the lower jaw as in the halfbeak [74] or a retarded growth of the upper jaws, as seen in the mandible of the swordfish [75].It is possible that the animal used its extremely long jaw to strike and confuse prey like other large animals with jaw elongation such as Eurhinosaurus [64] or Xiphias [76], and that these anterior teeth could be used to harm soft-bodied prey.
Arthrodires display a range of jaw morphologies, which reflect a variety of functions and adaptations, particularly in the Late Devonian [4,11,12,77].Some taxa continued favouring speed and strength, like Dunkleosteus [78].Others developed different types of feeding, such as the potentially filter feeding Titanichthys [13].More jaw disparity is visible in the Australian Gogo Formation with the durophagous Bruntonichthys, Bullerichthys and Kendrickichthys [14] and the supposed grazer Incisoscutum, with jaws adapted to catching soft resilient prey in the reefs or the water column [79].Like in Incisoscutum, Alienacanthus jaws do not appear to be adapted for high strength or speed, as fast and strong jaws were typically made of blades and reduced teeth [11,79].Although further information concerning the hyoid and hypobranchial apparatus could bring additional insights into potential jaw function, it is likely that such jaws would develop as an adaptation to a feeding or hunting method.This coincides well with the presence of recurved teeth on the gnathal plates.
The first gill arch of Alienacanthus can be compared to that of other placoderms (figure 6).The subtriangular shape of the ceratohyal is most similar to that of the pseudopetalichthyid Paraplesiobatis, although the latter may be more complex and composed of both the ceratohyal and first branchial arch.The lateral segment is larger and thicker than that of other early vertebrates, including that of Paraplesiobatis, and the chondrichthyans Acanthodes, Ozarcus and Debeerius [80].This structure may have provided strength and support for the elongated lower jaws.

Palaeogeographical context
During the Famennian, both the Holy Cross Mountains and the Anti-Atlas were part of the Rheic Ocean, connected to the Palaeotethys [81].McKerrow et al. [82] argued that the Rheic Ocean was small enough to allow migration of faunas between Laurussia (Laurasia and Baltica) and Gondwana.This is supported by the occurrence of similar taxa in both parts of the Palaeotethys [26,[83][84][85][86], indicating a close palaeogeographical relationship as suggested by Rücklin [26,87] for the Frasnian placoderm assemblages.The presence of Alienacanthus in both Gondwana and Baltica (figure 1a) concurs with the absence of a palaeogeographical barrier between Gondwana and Laurussia at that time.Together with the streamlined shape of the skull, this suggests that Alienacanthus was a pelagic swimmer, that inhabited much of the Palaeotethys.

Conclusion
The study of the anatomy, phylogenetic relationships and ecomorphological features of Alienacanthus is an example of how exceptionally preserved fossils can provide direct evidence on developmental evolution and morphospace occupation that would remain unsuspected and unknowable based on studies of extant species alone.
Ethics.No ethical assessment was required for the study.Fieldwork and fossil collecting in Morocco was obtained through permit no 1571/DE/DG.Data accessibility.The datasets supporting this article have been uploaded as part of the electronic supplementary material [88].
All authors gave final approval for publication and agreed to be held accountable for the work performed therein.
Conflict of interest declaration.We have no competing interests.

Figure 4 .
Figure 4. Eubrachythoracid phylogeny including Alienacanthus and simplified morphological diversity of skulls and jaws in different groups.Based on the computed 50% majority-rule consensus tree.Outlines of Dicksonosteus, Rolfosteus, Coccosteus, Dunkleosteus, and Melanosteus are modified from[8,[33][34][35], and[36].Comparison to other known taxa with an elongated lower jaw through time.Reconstructions not to scale and lower jaws are separated from the skull to show their shapes.

Figure 5 .
Figure 5. Live reconstructions of Alienacanthus.Based on the body morphology of extinct and modern fishes with elongated jaws (elongated, fusiform, bodies).Artwork by Beat Scheffold (Zürich) and Christian Klug.

Table 1 .
Measurements and sizes of main Alienacanthus specimens.(Estimations are marked with an asterisk ( Ã ) and incomplete in red.Values are in mm.(l) is for left element and (r) is for right element.IG, inferognathal; ASG, anterior superognathal; PSG, posterior superognathal.) royalsocietypublishing.org/journal/rsos R. Soc.Open Sci.11: 231747 jaw and show strong recrystallization but yield dentinous tissue, most likely semidentine, and a pulp cavity (figure